Biological Explanations Of Similarity.

Let’s start with the Tasmanian tiger. The Thylacine, also known as a Tasmanian wolf, or Tasmanian tiger, a medium-sized meat-eating mammal that as recently as the early 1900s lived on the island of Tasmania. Unfortunately, local farmers liked their sheep more than the thylacine, and eventually, they were hunted to extinction. The last known thylacine died at a Tasmanian Zoo in 1936. But that isn’t what makes the thylacine so interesting. It’s actually a marsupial. So are kangaroos, koala bears, wombats, possums, things like that. This is more closely related to a kangaroo or a possum than it is to the wolf that it looks like in its anatomy, shape, and size, etc.

Fossils and DNA tell us that mammals like wolves and mammals with pouches last shared a common ancestor about 160 million years ago. Another pair with a common ancestor that far back? A hummingbird and a Velociraptor. The thylacine is about as distantly related from a wolf or a fox as it can be and still be a mammal.

Consider the thylacine, their posture, the long legs, built for running, their skulls built for sniffing, and with massive muscle attachments for biting. And sure, they have pouches, but they look so similar to a wolf.

So how is it that two animals, this distantly related, could come to share so many similarities? Let’s have a look at the explanations of the similarity of these animals.

There are two biological explanations for similarity.

But there’s another explanation for similarity, and that’s called Convergent Evolution. Convergent Evolution, which means two animals their descendants have evolved to be similar to one another through the action of natural selection.

Human, a penguin and an ant walk to a bar, because we all have legs. If we go back to the common ancestor between human and the penguin, it already had legs, and they were built a lot like Human. We called this similarity as homologous because the trait was inherited from a common ancestor.

But the common ancestor between human and an ant was so long ago, it didn’t have legs. Our ancestors and ant ancestors evolved legs independently. We called these similarities as analogous. Analogous similarities are the result of convergent evolution. One of the most striking instances is in how the wolf and thylacine walk.

We, Humans, are plantigrade animals. Plantigrade animals put their whole foot flat on the ground. The ancient ancestors of the Tasmanian wolf were possums like human neighborhood possums and they’re plantigrade animals like we are, they put their whole foot flat on the Ground. When a dog stands, or a wolf stands, their heel is up in the air. The Tasmanian wolf keeps his heel off the ground, and it’s walking on its toes like a dog. Despite coming from different ancestors, the wolf and thylacine independently evolved this analogous trait: walking on their toes.

Another example: Take a bird’s wing and a bat’s wing. Homologous or analogous? The common ancestor of birds and bats already had a forelimb, so “having an arm-like thing” is a homologous trait. But using that arm-like thing as a wing evolved independently in birds and bats, so “flying with arm-like things” is an analogous trait. Birds flap their whole arm, but bats fly in a totally different way.

The scientific name for the group that bats belong to is “Chiroptera” which means hand-wing. So its wing is formed by its hand.

So, similarity inherited from a common ancestor is Homologous. And, a similarity not from a common ancestor is Analogous. So, let’s run down with the common examples.

The Human inner ear and Dimetrodon’s jaw joint are Homologous. Our distant shared ancestor had the same structure.

Platypus and duckbills are Analogous. Because their shared ancestor didn’t have it.

A bird’s feather and a reptile’s scale. They are different today, but both are inherited from a common ancestor. So they are homologous.

The wolf-like body of the thylacine. They are analogous. It formed independently, by convergent evolution.

The key to convergent evolution is a similar environment. Conditions like the climate, competition, and what foods around determine which traits are favored more than others. In the open ocean, a streamlined body and powerful swimming tail are proven to work, and we see it in all kinds of animals. Underground, a legless body seems to work pretty well. For medium-sized carnivorous mammals that have to run after their prey, a doglike body works really well.

And these are all just examples of similar body shapes and anatomy, but behaviors can be convergent. None of these are animals are best. There’s no such thing as “perfect” traits. Having a streamlined body is not the only way to be successful in the ocean, consider an octopus. And having a dog-like body is not the only way to be a successful hunter, consider a bear.

But even though evolution doesn’t march toward some ideal form, they show us an important lesson: Not all similarities are the same.